1. Field of the Invention
The present invention relates to an exterior environment recognition device for recognizing environment outside of a subject vehicle.
2. Description of Related Art
Conventionally, a technique has been known that detects a specified object such as a vehicle and an obstacle located in front of a subject vehicle to avoid collision with the detected specified object (collision avoiding control) and performs control so as to maintain a safe distance between the subject vehicle and a preceding vehicle (cruise control) (for example, Japanese Patent No. 3349060).
In order to perform, e.g., the collision avoiding control and the cruise control, a relative distance of the specified object located in front of the subject vehicle such as a preceding vehicle with respect to the subject vehicle is obtained. For example, such relative distance is obtained by causing two imaging devices (stereo cameras) having different stereo axes to capture a specified object, obtaining correlation between the two images, and obtaining the relative distance by means of stereo method (triangulation method) using parallax of the same specified object, attachment positions of the two imaging devices, and camera parameters such as a focal length.
In order to obtain the parallax according to the stereo method, it is necessary to correctly detect corresponding points representing the same specified object by performing pattern matching between images. As the pattern matching, corresponding points search method is generally employed, in which an image captured by one of the imaging devices is adopted as a reference image, and an image captured by the other of the imaging devices is adopted as a comparison image. In such corresponding points search method, any block (also referred to as a window) including one or more pixels in the reference image is set, and in the comparison image, a block highly correlated with the block (corresponding points) is searched. Then, the block with the highest correlation is adopted as the corresponding points, and the parallax of the coordinates between the blocks (the amount of displacement) is associated with the relative distance, and each processing is executed to perform the collision avoiding control, the cruise control, and the like.
In order to reduce the processing load in the pattern matching, a technique has been disclosed (for example, Japanese Unexamined Patent Application Publication (JP-A) No. 2008-39491). In the technique, during the pattern matching, vertical and horizontal sizes of the target image are reduced ½, and the stereo processing is performed on the reduced image, so that the amount of calculation is suppressed. A technique for reducing a search time is known (for example, JP-A No. H10-117339), by estimating a range of the parallax in accordance with the distance of the object and limiting the search range of the pattern matching to an estimated range.
However, even when the technique described in JP-A No. 2008-39491 is used, and all the specified objects including those at a close distance to those at a far distance are adopted as targets, it is necessary to have two images including an image for detecting a specified object at a distance and an image obtained by reducing the vertical and horizontal size by ½ for detecting a specified object closely located, and as a result, the processing load required for recognizing the image increases by 1.25 times. In the technique described in JP-A No. H10-117339, the size of the search range is changed in accordance with the distance to the specified object, but when the specified object is located in proximity, the processing load is not reduced, and in the loop processing for repeating the same processing with a regular interval, it is necessary to ensure a time for the processing load. Therefore, the processing load cannot be reduced drastically.
Accordingly, the inventor of the present application has found that it is possible to stably detect even a specified object located in proximity while the processing load is maintained, as long as only the position of the search range (the amount of offset) is changed without changing the size of the search range of the corresponding points in accordance with the distance to the specified object. However, when the search range of the comparison image is offset by the amount of offset in order to change the amount of offset of the search range, the parallax of the specified object at a far distance may be reversed at the right and the left, and in such case, the specified object at a far distance, which should be detected, cannot be detected. When, under circumstances where occurrence of an undetectable area due to the offset as described above is expected in advance, the pattern matching is performed for that area, this may cause false detection and increases the time it takes for the processing, which unnecessarily increases the processing load.